The present invention relates to apparatus for the regulation of speed and frequency, in general, and more particularly to uninterruptible power supply (UPS) motor drives and/or electrical generators operable at a fixed frequency.
The present invention will be described more specifically in the context of a motor-generator set in which a separately excited DC machine is coupled to a synchronous AC machine for the generation of AC current of a standard frequency, for instance, 60 Hertz. This type of generator is particularly suitable for the power supply of a ship where a DC power grid is available which needs to be tied to an AC power grid for interconnection with transfer of power in either direction. This is the situation encountered when a motor-generator set is used as a standby, which can, in an emergency, be introduced to provide auxiliary AC power at a fixed frequency on the AC grid, in the absence of any generator of sufficient capacity to impose a reasonably constant frequency all the time. The fixed frequency, for instance 60 Hertz, is maintained by adjusting the speed of the DC machine coupled to an AC generator which is a synchronous machine. The constancy in frequency of the generator thus depends upon the degree of accuracy in the speed regulation of the motor. Frequency adjustment is obtained with a frequency representative feedback signal compared with a speed reference signal to compensate for any error.
Two approaches have been used in the past to generate such a feedback signal. One has been to convert frequency into voltage. The frequency to voltage (F/V) converter then becomes a crucial element in achieving regulation with accuracy. Any error introduced by the F/V converter cannot be corrected by the closed loop regulator. This raises a problem when the implementation of a F/V converter requires various inductor-capacitor networks, since they are difficult to manufacture with a high degree of accuracy and are sensitive to temperature.
Another approach has been to make use of an accurate time base, such as provided by a crystal oscillator, to measure the period of the AC line, e.g., the inverse of the frequency. However, when so doing, the prior art has used digital techniques requiring signal conversion into analog for the purpose of comparison with the AC period. Here also, like with the frequency-to-voltage converter, the digital-to-analog converter is a source of error.
The present invention allows for the use in a closed-loop of both analog reference setting and analog feedback, while taking advantage of digital techniques.